All right, in the last lecture, we modeled the sky with a big sphere that we called the selectional sphere, and we basically made fixed points on that selectional sphere, which is the north. Selectional Pole, the South Selectional Pole and the Selectional Equator. With these are basically online and what we have on the surface of the Earth. And we said whatever you're, wherever you're located, you basically have a horizon and that horizon you don't have a choice in choosing your north.
Your north has to be below the North Selectile Pole. And once you find where the North Selectile Pole is, which basically you can look for Polaris, you can measure your latitude L. And once you know your latitude, you can decide, and we explained that in the last lecture, that you can only see 90 minus L from the southern sky and the rest will be from the northern sky.
and we said if you're in Al Ain, your latitude is 24, so you can only see about 66 degrees from the southern hemisphere, or the southern sky, and totally see 180, so you will see 114 degrees from the northern sky. And then if you are somewhere and you want to basically decide on your latitude, you try to look at the little dibber and the bigger dibber, and then you can look at Polaris once you decide on that. And if you do not have any tools, I told you you can use your hands to decide on your latitude. And I think we stopped here at this slide here. And in this slide here, we're just trying to give you a different latitudes.
And again, the latitude is the angle that measures between the northern horizon, that means the north and your horizon, and the north selection pole. So for example, this is basically now your latitude here, and in this case you have what you have 30 degrees. That's what your latitude is. Now, in some notation, this is used as 30 north.
That means you're basically measuring this relative to the northern horizon. And, you know, if I give you this argument, that means 90 minus 30 will be 60. That means you're going to see 60 degrees from the southern sky, and the rest, which is 120, will be... be from the northern sky. Now, below this one here, in this one here, actually you are in the southern hemisphere. You are not in the northern hemisphere.
And when you are in the southern hemisphere, the north celestial pole will be below. You're not going to see that, but what you're going to see is you're going to see the south celestial pole. So you're not going to see the north celestial pole if you are in the southern hemisphere. And in that case if you want to use or measure your latitude then you're going to measure the angle between the southern or the south celestial pole and the southern horizon or the south and your horizon. Now here we say it is minus 30 degrees because actually if you want to do it, it the north celestial pole will be here.
It's below your horizon. But in some cases, they look at that as 30 degrees south. They put s here. That means it's measured relative to the southern horizon.
Again, by the way, if you want to use the rule of 90 minus L, so it will be 90 minus minus 30, so it will give you 120. So you'll see 120 degrees from the southern hemisphere and you're only going to see 60 from the northern hemisphere which is the same argument that we talked about which is logical. You still can use this argument as well. It works even if you are in the southern hemisphere.
Now there are two special locations and one location is latitude 90 degrees. And what do we mean by latitude? 90 degrees.
Now, if the northern or the north selection pole is at your zenith, which is exactly on top of your head, that means you are exactly in the north pole. If you are on the north pole, the north selection pole will be exactly on top of your head, and this will be your horizon. So this angle here will be 90 degrees. And that's why we say you are at latitude 90 degrees. So latitude 90 degrees, that means you are at the North Pole.
And in that case, if you are at the North Pole, you're only going to see the whole northern sky because this will be also the selection. equator, which will cut it in half. So if you use the argument, which is 90 degrees minus L, and L here will be 90 degrees, so you will get zero.
So you're seeing nothing from the southern sky, and you see the whole northern sky. Now, if you are located at the equator. And if you are at the equator, at your zenith here will be the selectional equator. But the north selectional pole will be here, will be on the horizon. That means the north selectional pole will be on your horizon, which is the same as the north.
So the angle between both of them are going to be 0. And in that case, you are at latitude 0. So when you are at the equator, you are at latitude 0. When you are at the pole, you are at latitude 90 degrees. And if you use the argument here, which is 90 minus L, so L here, it will be 0. it will give you 90 degrees. So you're seeing 90 degrees from the southern sky and of course you will see 90 degrees from the northern sky which makes sense. If you are exactly at the equator, half of your sky will be in the southern sky and half and the other half will be in the northern sky. And that's basically, so when you go to different location, you should not be surprised of what you see in the sky.
This is because of the different latitude. Is this clear? Is this clear? Good. So now, what happens?
when you look if you are at these locations, and we'll discuss this a little bit further, if you are at latitude 90, that means you are at the north pole, that means the NCP, which is north celestial pole, on top of your head, and the observer is here. Now, if you are at that location, actually the sun will go around you. And we'll talk about that later when we talk about seasons. You will have six months of daytime and you will have six months of night time. But we'll wait till you get to that.
But that's basically how the sun is going to go around you. It never sets if you are in this period of six months. You are somewhere like us.
We are not at the equator. We are not at the pole. We are somewhere at 24. So during the day, due to the rotation, so that's basically the north selectile pole, and that's how the Earth is rotating around that. It's rotating this way. That's the observer.
So the observer will have a north, which is exactly underneath the north selectile pole. The south will be there. That's west, and that's east. And you know that the sun comes from the east, it rises, and then it goes where?
It goes toward the west. Now the sun basically doesn't go in a straight line up, it basically it's tilted that way. So during afternoon time you may actually see it in or even at noon time, it may actually go toward the south.
So if you look at the sun in our location here, very early in the morning, if you can spot shadows, the shadow will be in the west because the sun is in the east. And if you look toward where the sun is going to end, the shadow will be toward the east. But if you look at noontime and around, you'll find...
the shadow will be in the north. The shadow will be pointing north because the sun will be basically tilted toward the south. Now, if you are on the equator, now on the equator, this where will the north This is where the north celestial pole is going to be and the south celestial pole. That's the rotation. You're rotating this way.
So actually the sun will come from east west. So at noon time it will be exactly at the highest point in your location. So that basically happens if you are at the equator. So you have to be careful when you try to look at the Sun and you try to look at directions. If you are in the North Pole, it's very difficult.
If you are in the equator, the Sun basically goes east-west and that's what most people think. But in a location like our location, no. The Sun, if this is basically east, if this is east and this is west, and you expect the sun goes this way, it doesn't go that way.
Basically it goes and it's tilted toward what? Toward the south. So with this, you really have to be careful of how to look at the night sky due to the rotation of the earth. During the day, you only look at the sun, but during the night you actually can see stars.
Now what you see in this image here, it's not one time, it's what we call time lapse. That means many photos are put on top of each other. And you can see here that the object is, or the object is rotating around.
Why? Because you're basically, this is the way the Earth is rotating. So you see the stars basically doing this. Now, if you are at the equator, and remember what we talked about the equator, you're actually going to see, and again, I tell you this is not, there are no, you cannot see this streaking at night.
but it's basically you take photo and then you put the next one and the sixth one we call this time lapse and you see it basically going what going up or it may actually go down that's it due to the rotation of the earth and if you are somewhere in the middle actually the streaking or the time lapse of the star motion it will be something like this it will be tilted okay And it's not going to be where you look at the North Pole, where it goes this way, or the equator. where it goes this way, you're somewhere in the middle, so you're basically tilted that way. So in general, if you are, if you view things from the North Pole, you will see basically the stars going around this way because that's your axis of rotation here.
If you are at the equator, the axis of rotation is this way. That's where the North Seletcher Pole is and you'll see things basically going this way. So up and down that way.
But if you're somewhere in between, that means maybe the axis of rotation is this way, then you see things are basically going this way. And that's what we say here mid-latitude. So even when you look at the sky and you try, let's say, looking at the sun during the day. You have to be careful of where your latitude is. And if you want to look at stars, especially this is at night, and we'll talk later on when we look at different direction in the sky.
We'll try to explain this shortly. You can see these different effects. So your latitude dictates what you see.
So now let's say on your horizon. If you have a horizon and in your horizon you have the north, you have the south, and then west and east. You can stand in your horizon here and you can look at different direction.
You can look north. Of course, when you look north, you're going to look up. Or you can look south or you look west and you look east. What will you notice? of some feature in the sky, let's say you take a constellation and you look at it, what will happen to it during the night?
So if you look north, and that means you're not going to look north on your horizon, you're going to look north up, and that's exactly where the north celestial pole is, and that's the direction of the rotation. So if you look for example at the little dibber and the bigger dibber. And that's another name here. We say Ursa minor, it's for the little dibber and Ursa majoris is for the bigger dibber. And remember what I told you about the North Star, which we call Polaris, which is exactly very close to the North Star, celestial pole.
So if you take an image of that and you try... to put it one photo after the other you actually see it going around this way so it rotates around now again with this you can tell time because you can basically look at how this spoon or the shape of the spoon looks like and you look at the its location And with this, you can basically tell time and it can help you to navigate as well. So looking north, you can see things basically moving this way and we call here it's circling counterclockwise.
Now if you have a clock in front of you, you know that the clock basically goes this way. Counterclockwise is the other way and that's what you're going to see in the sky. That's how the object is going to move through the night.
But you have to look where? You have to look north. So now there are constellations that we call circumpolar constellation. And circum comes from circumference and polar comes from pole.
So these are the ones that are basically around the North Pole. That's where Polaris is. So, this constellation, during the night, because of the rotation of the Earth, they will be basically rotating counterclockwise, or revolving counterclockwise, and we call this the Circumpolar Circle.
And that's how this object will move. during the night. And this is where you can basically, if you're familiar with the sky and so on, you can tell what time and it may also help you in directions as well.
That's if you look north from your direction. Now, what happens on my horizon if I don't look north? I will try to look east, okay? And of course, you're going to look up.
Now, the Earth is rotating around this. axis here. But when you look toward the east, you will see that whatever constellation you look at, it will start to rise up in the sky.
Again, the line that you have here, that's not a line that you would see, but that's if you put one photo after the other. And of course, one of the famous that we talked about in the last lecture, Orion constellation, it will be rising up when you look east, if you spot it in the night sky. And of course, again, depending on its location in the sky, how high, how low, you can tell time, and you can also tell direction, and so on. Now, if you look south in your horizon, and of course you're going to look up to the sky.
You're not going to just look ahead of you. And what you will see, you will be basically seeing a constellation like Ken's measure here will do something like this. It will have an arc or a trajectory like this.
This is if you are in the northern hemisphere. During the day, you have no choice. You only can look at the sun.
But during the night sky, you can look in different directions. And if you spot any of these constellations, you can basically follow them during the night. So if you look north, you will see basically the constellation going counterclockwise this way during the night. If you look... East, you will see the constellation rising up in time.
And if you look south, you will see something like this. That's if you are in the northern hemisphere. If you are in the southern hemisphere, the situation will be different.
Because in the southern hemisphere, the... the south celestial pole is above your horizon, and that's your axis of rotation now. So to see this effect, and if you are in the southern hemisphere, you're not going to look north. You're going to look where?
You're going to look south. So if you're on southern hemisphere, and you want to see this effect here, of course, you're not going to see the little dipper. You're going to see something different.
what you will basically have to look south to see this. Now if you are in the southern hemisphere and you see rising like this, well you're not going to look east, you're going to look west. And if you want to see this trajectory here, then you're not going to look south, you're going to look north. on your horizon, if you are on the southern hemisphere, to see these effects.
So that's the difference between how to look at the sky if you are in the northern hemisphere or in the southern hemisphere. And I told you if you look at these constellations and you spot them, they can basically tell you time at what time of the night you're in and you can also tell directions as well. And this is similar to what you can see during the day time.
Now, today is May 26. Today there is a total lunar eclipse, but you're not going to be able to see that in Al Ain. You have to be in North America or New Zealand to see that. However, we'll see something that we call a super moon and we'll talk about this in chapter number 8. and we'll talk that when you have a full moon, the full moon starts from sunset and it will be in the east and then it will be at the highest point around midnight and then around sunrise or when dawn.
the full moon will be in the west. Of course this is due to the rotation of the earth and that's why I told you if you want to see the moon or the full moon bigger and reddish and so on you do that exactly after sunset and then if you wait and look three four hours two hours after that it may actually look smaller to you and may get a little bit bluish If you want to see it at the highest point in the night sky, it will be around midnight. And then after that, it will go toward the west. And that has to do with the rotation of the Earth. And that's how you can tell time and you can tell direction as well.
So you can use the moon, you can use the stars to look at it at night to tell basically direction and time. Any question on this? Is this clear?
Any question on this? That's basically how you can view the sky. You decide on your latitude and then how to view things relative to your latitude and how much you can see.
Now we're done with this. Now we're going to turn our attention now to the Earth itself. how or the motions of the earth, how the earth is basically moves.
Now you all know that the earth revolves around the sun once every year and we call this a revolution and we also have a rotation which takes one day around its axis of rotation, which is basically the axis that connects the north to the south. And we call this the rotation. Now, if the Sun is located here, And the Earth is basically revolving around the Sun. Now this line here, the dashed line, is perpendicular to the orbit of the Earth. But the North Pole and the South Pole, where you have the line which connects them, which we say that this is the axis of rotation.
This axis of rotation is tilted and it's tilted at an angle of 23.5 degrees relative to this line which we call perpendicular or normal. So that's what the earth is doing. It is revolving around the sun and we call this one revolution. It is rotating around its axis of rotation, and that axis of rotation basically connects the North Pole to the South Pole. Nowadays, if we want to know where the North Pole is, we have to look at Polaris, which is very close to the North Selectoral Pole, and the North Selectoral Pole is on top of the North Pole.
But the question is, why this tilt? Why the tilt of 23.5 degrees? And what effect will that have? Now, when you have this tilt, we say that the axis of rotation is also going to precess. And we call that precession.
And this precession will basically precess around the normal axis. So, Procession only happens when there is rotation. So it doesn't happen by itself.
There has to be a rotation. And this will happen. So let me just go here and try to show you this.
Okay. Now this is what we call a top. And the top, it doesn't have a regular shape. It has an iconical shape this way.
So if you look at the point here, to this point there, that's basically the axis of rotation. And this, the top, is basically rotating around this. But at the same time, you see this axis of rotation is wobbling around. And this is what we call, what we call, precession. So...
Now, if you look at the top here, the top, due to gravity, will try to fall this way, but it doesn't fall. It keeps up only if it is rotating. So to offset this falling down, it will go to the other side. So it will be here.
So if you take the axis, which is normal here, you will see that this axis of rotation here will go around this way. And this is what we call, what we call precession. So the earth is rotating and that axis of rotation is not normal.
It basically makes an angle of 23.5. So this will precess and it goes around this way. And it will complete one full precession every 26,000 years.
That's slow. Now the question that it's asked, why the earth is tilted? Well, when you look at the top here, the top is not uniform.
So for example, If I take here a ball like this, the ball is, if it's a sphere, it will be the same in all directions. So it will be basically rotating this way. It's not going to tilt. But when you look at the top here, the top is not uniform.
It has more hair and less hair. So a gravity from the Earth will pull more and the mass. Now when we look at the Earth, the Earth itself is not really a perfect sphere.
It has bulges here. Bulges in Arabic means that it is moving around the equator. Now the Sun pulls on the Earth, the gravity of the Sun.
And the Moon also pulls on the Earth. So what will happen? It will...
tilt the earth the same way the top is tilted. But since the earth is rotating, this axis of rotation basically precess. But if the earth is a perfect sphere, it's not going to be tilted because the pull will be the same in all location, the same thing that will happen from the moon. So you'll actually have it. with zero tilt.
But since it has bulges around the equator, this gravitational pull from the Sun and the Earth tilts the axis of rotation, and now the tilt is around 23.5 degrees. And since the Earth is rotating, that makes the axis of rotation precesses, and it makes a precession. and that procession takes 26,000 years.
So here we can say that the Earth revolves around the Sun and it makes one full revolution every year. It rotates around its axis of rotation and it does that once every day. and it processes around the vertical or the normal to the orbit of the Earth, and it does that in 26,000 years, which means the North Selectile Pole will go around.
Remember what I told you right now, if we want to find where the North Selectile Pole is, we have to look for Polaris. But in time... this North Selection Pole will basically look at different objects.
And if you recall in the last lecture, I told you you have to look at Vega and the constellation of Lara, in 12,000 years from now, the North Selection Pole will be pointing there. And that also will affect the way we look at the sky. And the tilt that we have now, it's 23.5, actually it varies between 22 and 24 as well.
So the Earth has some motions, okay, where the variation in the tilt, the precession, the rotation, the revolution, and we'll talk about later also the orbit itself or the elliptical orbit of the Earth also has some variation as well. And these will have some effects on what we see in the night sky and also our seasons and so on. So the precession, again, I told you it comes as a result of the tilt of the axis, as you can see here, and the top is not really uniform, so the gravity from the pull will try to bring it down, and of course it can resist falling down. by precessing to the other way.
It's like Newton's third law. To every action, there is opposite and equal reaction, so it will go to the other way. And that causes what causes the precession. Is this clear?
Is this point clear? Now, there is something that we call gyroscope, and this gyroscope is a tool that used to balance things. Now, for example, you have something that known as the middle ear and that's what keeps you balanced.
You can walk straight but when you get dizzy you start basically wobbling and a procession as well sometimes is referred to as wobbling. So this gyroscope can basically a tool be used to correct for the Wobble, so even a precession as well, sometimes is referred to as wobbling. And that's what happens when you lose your balance.
You get infection in the middle ear or so on, and you start basically wobbling. So we say the Earth is revolving, it is rotating, it is wobbling at the same time. Are these points clear? Any questions on these? So these are basically the motions of the Earth.
What will be the effect of the precession? You have a precession, and that precession basically takes 26,000 years to complete. So your axis of rotation, or the North Selection Pole, will be basically varying. Now, around 2100... Polaris will be at the closest to the north celestial pole.
It's not exactly there, but at this time it will be almost close to it. But because of the precession, this will move around, this axis of rotation will move around. In 12,000 years from now, if we want to locate the North Selectile Pole.
I'm not going to look at Polaris, I'm going to look at Vega. I remember what I told you in the last, if I spot Vega 12,000 years from now, then actually it's in the constellation of Lara, then you're actually looking at the North Selectile Pole. So over the course of time, what you see in the sky will be a little bit different. Okay, so we have to be careful with that.
Of course, that doesn't happen in a human's lifetime, but you can have data recorded and so on, and you have to look at that. So here we say around 2000, that's where Polaris is. Now, this is where Vega is. You can see in this, it tells you 9000 BC.
That means before Christ or now. this is 2000 before, now are here. So as you move around, your north celestial pole is not going to be Polaris, it's going to be something else as you go around.
And that of course will have what will have on effect and what you see in the night sky. So the earth is revolving, rotating, processing, or wobbling and these will have effect over the course of time. Now when we look at the cycle that caused by the sun, one thing is basically the earth's rotation and that's basically when you get day and night every day and the cycle comes because of the rotation of the earth. So this rotation of the earth is basically giving you the day and night. So at noontime you would see the sun basically at the highest point and this is when the sun rises and that's when the sun sets and of course then you go into the night.
The sun is not doing that but the earth is rotating and it's causing this. So this is basically one feature of how we look at that. Now the Sun or the Earth is revolving around the Sun.
And remember what I told you about the Earth. So when you look at the Sun, for example, here, and you look at the Earth here, this is the orbit or the line that connects the Sun to the Earth. Now, the axis of rotation of the Earth is tilted at an angle relative to the normal or perpendicular to this line here.
Now, we live here on the Earth, and we look at things relative to us. Now, I want you to imagine the following. If you are in a car, and you are on a highway, and there are trees.
beside the road or the highway and the car is speeding up and you start looking at the trees, you may feel that the trees are moving. But if someone is outside, he will say, no, the trees are not moving, the car is moving. But relative to you, you may feel that the trees are moving. The same thing happens if you look at a car ahead of you. If the car maintains the same speed as your car, you may think the car is not moving.
It's stationary because both of you are moving at the same speed. But if the car is moving faster than you, you will feel the car is moving ahead of you, but only moving with the difference between your two speeds. But if you're moving faster, you feel that the car is going backwards relative to you. So the same thing happens here.
We look at things relative to us here from the earth. And if we look at what we have in this cartoon here, the earth is basically revolving around the sun. But relative to us, we think that the sun is revolving around the earth during the year. But because of our tilt. we see the sun basically moving this way.
Now if you look at this angle here, you will find this angle is 23 and a half degrees. And of course, since you will be tilted, but you will be moving around, sometimes you will be tilted toward the sun, sometimes you will be tilted away from the sun. And that is a cause of seasons, and we'll talk about that different.
So now, When we talk about the path of the earth around the sun, we call that the ecliptic. In Arabic, we call it the circle of the sun. I'll explain that shortly.
Which, it has two definitions. The first definition, I'll take the second one, which is the true path of the earth around the sun, which is this one. that's the actual definition, or the apparent annual path of the Sun around the Earth.
And that's how we see the Sun is basically moving around us. So we can see points here where the Sun is very high above the celestial equator, a point where the Sun is below, And sometimes the sun crosses the selection equator at these points. And we'll talk about this later and the effects on the seasons.
Now, this is what the ecliptic is. Now, during the day, we see the sun. And as I told you from our location here, the sun doesn't basically go.
east-west. It actually goes from the east but it goes toward the south and then it will fall to the west. So this is basically the ecliptic. How did we arrive at this ecliptic?
And this was known a long time ago. And how did we trace this? During the day it's easy to see the sun but how we look at this revolution and it came as looking at certain constellations in the sky. And these constellations sometimes happen to appear at certain times of the year.
And that's why they created the telescopes, which in Arabic we call them Al-Buruj. And each one has a certain time. You can basically look at it as pizza. You can basically look at this.
as a pie or pizza, you can basically slice it this way, and then you can slice it this way and that way. You can basically have slices, and these basically you can look at them as a time where some of these basically appear. And you can look at these times. So for example, you have here different constellations.
Sometimes we call these the zydochs, and you can look at them. And we can look at the time they appear and then they disappear. So the way we look at that, we say the sun should be there around that time.
And that's how they actually came with this ecliptic. So we say this is where the sun actually should be and that's how we traced out the ecliptic and I told you in Arabic maybe the word burj it could be not really something a tower but it could also be a piece of like this could be a slice and pizza or pie or best if you have a watermelon and you can break the watermelon and things like that that could be regarded as a Burj. So now you basically in this slide here, you can look at the times and you look at the constellations that you have here. So now we say that this is where the Sun appears. So you see this.
horoscope during the night. So you say this is where the sun should be and that's how you trace how the sun is basically moving around the earth. Now that's the ecliptic.
So for example, you are on the Earth and you have to look at the Sun. So if you pick, for example, we are now in May. So if you take May 21st, then opposite to you is the Taurus, which is this horoscope here. That's what you're going to see now.
and it has a certain time. This is where we say the Sun should be during the day, and you should basically start tracing this. So now, long time ago, these constellations, they appear in certain times. Okay, so for example, we are going to talk about May 14 to June 19, and that's the Taurus, which is a Thor, and this is the time where you can see this constellation, and as I told you, this was used to trace out the ecliptic, or the Sun.
Now, if you count these, you'll find these are 13, not 12, and these are the times where they appear, but you have to be careful. Remember what I told you about the precession of the Earth. So these will change in time.
And remember what I told you in the previous lectures about astronomy and astrology. So astrologists, they look at these horoscopes and they look at other alignments and they try to come up with... what affects humans. Actually, there is no connection. They use these.
And I thought these actually will change over time as well. The times will be different due to the recession of the Earth. And as I told you previously, that astrology is ilm al-tanjeem, astronomy is al-falak, which is different. And nowadays, since people use every way to scam people of money, you find someone on TV presenting themselves as falakiyyin and they try to tell you what your horoscope is and what will happen to you and it depends how much money you're going to pay them so there is a big difference here.
A long time ago astrology was considered in Babylonian times and so on and they thought it has effect who's born today is going to be a great person or a miserable person and so on things like that but These horoscopes were basically used to tell of the ecliptic. Any questions so far? Any questions? Any questions?
So that basically the motions of the Earth, revolutions, rotation, and precession. And then we define the ecliptic. And the ecliptic is the actual path of the earth around the sun or the apparent path of the sun around the earth.
And since you are on the earth, how can you basically decide on that? I told you you can decide on that from the zydeck or the horoscopes that you look at it during the night and assume that the sun is there. Now, we experience seasons here on Earth, and we want to know what causes these seasons.
And one quick argument that people can always get to, they say, well, in summer it's hot, in winter it's not as hot. So they say, well, in the summer we are close to the sun or the Earth is closer. to the sun and in the winter the earth is further from the sun and so on.
But I think I told you that the average distance between the earth and the sun is about 150 million kilometers and probably the variation in it is not going to be that big to produce that effect. But what actually responsible for season is the tilt. Now it's 23.5 degrees.
That's what basically causes the seasons. So as the Earth revolves around the sun, and it rotates around its axis every day, this tilt here, it could be toward the sun or away from the sun. And that could be the reason for...
seasons. So, for example, and remember when we talked about the, let me just take you back a few slides, when we talked about the ecliptic, and if you look at the north celestial pole, the south celestial pole, exactly in the middle, you have the celestial equator. Now, because of the tilt of the axis of rotation of the Earth, That's how the sun looks to us. So the sun sometimes is above the celestial equator, and sometimes it's below, and sometimes it is basically crossing the celestial equator. So when the sun is high, that's when we're going to have summer.
And when the sun is below, that's when we're going to have winter. And that's why during the summer time, you see actually the sun is a bit higher in the sky at noontime than the winter times. So now this is where if you going back to what this is the selection equator and that's where the sun is during the time where the earth is tilted toward and you will see the Sun at noontime at higher location in the sky. When it gets to the highest point, this is where we call Summer Solstice, meaning the beginning of summer. And at that time, when it's the lowest point below the celestial equator, this is when you have the Winter Solstice, which is basically the beginning of winter.
Now, if you want to use the argument that we are closer or further away from the Sun, now we know that we have opposite seasons if you are in the northern hemisphere or if you are in the southern hemisphere. So the argument is the Earth will be at the same distance from the Sun. for both the northern hemisphere and the southern hemisphere.
Yet, at the same time, when there is summer in the northern hemisphere, there is winter in the southern hemisphere. So, the idea of you're further or closer to the sun, it doesn't work here. But it's actually the tilt.
And you know the axis of rotation. If we are here in the northern hemisphere and we're tilted toward the sun, then in the southern hemisphere, they're tilted away from the sun. And in the winter here, when we are tilted away from the sun, in the southern hemisphere, they will be tilted toward the sun.
And of course, at that time, they will have what they will have, summer. So that basically depends on the sun rays that strike you here on the Earth. So for example, if we are tilted toward the sun, this is where we have a steep incidence. That means you're getting really sun rays that's hitting you at steep angles.
Now, if you are in the southern hemisphere, you're basically tilted away and you're getting here shallow incidents. And that basically may affect you in receiving less sun energy. And that basically not going to give you the effects that you can see in the.
in the seasons. By the way, the atmosphere is important for this. We have what we call the greenhouse effect. So it traps energy or heat that we get from the sun. That also has a factor in there.
But the main reason that we see these seasons on the earth here, it's due to this. It is due to the tilt of the axis of rotation. And I told you this tilt comes because the Earth is not a perfect sphere.
It has bulges around the equator and that the gravity from the Sun, the gravity of the Moon basically tilts this axis of rotation which causes the Earth to process. So we're basically that plays role in the seasons. But if the Earth is perfect sphere. then the axes of rotation are not going to be tilted. And what you will note, you'll basically note the same season at your location all year round.
So, more or less, you're going to have one season, and you're not going to have the precession of the Earth. But the Earth is not a perfect sphere. So, here, this is what I told you previously.
When the northern hemisphere is tilted toward the sun, you have summer, northern summers. Of course, the southern hemisphere will be tilted away, and that's when you have southern winters. Now, when you're tilted away, then in that case you have northern winters, and of course those in the southern hemisphere, they will have a southern summers and that's why we have two seasons at the same time on the earth one in the northern hemisphere one in the southern hemisphere. Are these points clear? Are these points clear?
You can chat you can also use the mic. So now you may say that in some location we experience four seasons. And what's causing this?
Of course, your position relative to the tilt, your tilt relative to the sun. So when we look at our celestial sphere, in the celestial sphere, that's where the Earth is. Now, this is basically the celestial equator, the blue plane here. which cut the celestial sphere in half.
Then you have the goldish plane here, and that's basically the ecliptic of how the Sun is moving. So when the Sun gets to the highest point here, relative to the celestial equator, where it actually makes an angle of 23.5 degrees with it, if we are here in the northern hemisphere, we'll take things relative to the northern hemisphere, This is when you actually get summer. And when you are basically at the lowest point below, the ecliptic below the selectional equator, that's when you're going to get winter. There are two points where you're basically crossing, the ecliptic is crossing the selectional equator, and we call this equinox.
So when you leave winter and you're going toward summer and you cross this point, this is when you basically get into spring. And then you go toward the summer. Actually, we're still in spring and we're going toward.
summer. We are not here. We're maybe somewhere around here, okay, to get to the summer in the northern hemisphere.
And when you cross summer going into the winter, well, you have another equinox, but that time we call it autumn, and there is another word for it called fall. There is another word for spring called vernal as well. And that's how we basically get the seasons. That's three-dimensional cartoon here to show you.
when summer starts, when winter starts, when fall or autumn starts, when spring starts, and so on. We can do that two-dimensional, which you can draw by your hand. This is where the north celestial pole is.
This is where the south celestial pole is, and that's basically your celestial equator, exactly in the middle. Now you draw your ecliptic and the ecliptic makes an angle of 23.5 degrees relative to the celestial equator. Now when the sun gets to the highest point above the celestial equator, that's SS, that means summer solace. And when the sun gets to the lowest point, that's when you have the winter solace.
When you're crossing. when the ecliptic is crossing the celestial equator, that's when you have vernal equinox or autumn equinox. So it is the tilt of the axis of rotation of the Earth that is responsible for seasons.
But again, remember I told you this actually precesses. So in time, we may actually reverse this. So now we have summer in the northern hemisphere, winter in the southern hemisphere. We're very close to that.
In time from now, when the axes basically go the other way because of the precession, then the seasons will be reversed. So we have to be careful on the long run of how this will happen. Now, I'm not going to get into the calendar and how it was basically developed a long time ago.
It was discovered that, especially in the era of agriculture, it was important that we have to know the seasons because cultivation happens at certain times and planting things at certain times and so on. So the... calendar was created in such a way that you actually go through this seasons. So more or less because remember when we talk about a long time ago it was decided that day and night that's basically when you get day and night and that has to do with the rotation of the earth so we will live at different location on the earth so you may have differences in time and so on, so we have to keep that in mind.
For the spring equinox, when the Sun crosses the celestial equator, that means the ecliptic when we talk about this, that usually happens around March 21st, and when the Sun gets above the highest point above the celestial equator, which is summer solstice, will be around June 21st. very soon we will be entering summer and autumn aquanets around September 23rd and winter solace will be around December 22nd. That's if you are in the northern hemisphere.
In the southern hemisphere it will be opposite for you. So these are plus or minus. I mean when it comes to these days to decide and what time and of course I told you about time and date side that I showed you. You can go there and they will tell you at what time the summer will begin at your location and so on and winter and all of these things.
So we have seasons because of this. Now A long time ago, when the Egyptians and the Babylonians, they wanted to decide on the time, so they actually looked at the sun, and the sun basically gives you daytime, nighttime, so that's a day to them. But how to look at the time, so basically they start looking at noontime.
where the sun basically at the highest point. And that's why you have AM and PM. Okay.
It's coming from Latin. I forgot the Latin words for it. But there are people who used to go and look to see when the sun is at the highest point. And that's basically noon time to them.
So noon time became somehow important. to look at things. So noon time basically when the sun should be at the highest point in the sky, which is we say crossing the meridian line.
So in different locations, something was noticed to signal the beginning of the seasons. So we're going to look at 23.5 north, 23.5 south, 0 which is the equator, 66.5 north, and 90 north. These basically have, things have basically noticed on the first day. So if you are on latitude 23 and a half, and this is called the Tropic of Cancer. When in here, we happen to be at 24, but you can, you can travel to this.
Okay, you can basically go to that location. Now, what was what notice that on the first day of summer, the sun will be at the zenith, at the highest point in the sky. So on that day at noon time, you're not going to see a shadow.
Your shadow here in Al Ain at noon time, when the sun will get to the high point, it's going to be pointing north. I told you because of how the sun is moving. But it will be the shortest distance.
And that's what we say that this is now a dhuhr time for salah. But in summer. when the sun is exactly on top of your head, actually you have no shadow and that's Zawal time, that means this signifies the beginning of Dhuhr time.
So that's something notes if you are at this location. Now if you are at 23 and a half, South. Well, it's notes that the, which is on the first day of winter, the sun will be where? Will be at the zenith point. And this has to do with the tilt, by the way, of how the earth is tilted.
Now, if you are at the equator, the sun will be at the zenith on the first day of spring and the first day of autumn. That's when you see the Sun basically at the zenith point. Now if you are at the equator, that's interesting, if you are at the equator, What you will notice that on the first day of spring to the last day of summer, the sun never sets.
You have six days of daytime because now the ecliptic is crossing the celestial equator and it's above the celestial equator. And six months of night from the first day of autumn to the last day of... winter. And this is something basically you will notice if you are at the North Pole. Of course, the opposite will happen if you are in the Southern, in the South Pole.
Now, at 66.5 North, which like Scandinavian countries and so on. On the first day of summer, you have 24 hours of night daytime. And on the first day of winter, you have 24 hours of night time.
And this is something that has been noted at these locations. So in some places, they can basically mark the beginning of seasons when this happens. Now, when you go to higher and higher latitudes, in the summer times, the days get longer and the nights are shorter.
And when you go in the winter times, when you go toward higher and higher latitudes, the night time becomes... longer and longer and the day time becomes shorter and shorter. Of course, for Muslims, this can pose a challenge when it comes to fasting Ramadan. So I think I presented here the causes of seasons and what are the seasons and the special effects that you can see at different locations.
Are there any questions on this? Is this clear? You can use the mic as well. Is this clear?
Okay, good.